N-port circulator

ABSTRACT

This application describes an n-port circulator comprising n identical circuit units arranged in a closed loop configuration. Each unit is a three terminal circuit including two transistors and two equal, impedance-matching impedances. The first terminal is connected to the base of the first transistor, and to the emitter of the second transistor through one of the impedances. The second terminal is connected to the collector of the second transistor, and to the emitter of the first transistor through the other impedance. The third terminal is connected to the collector of the first transistor, while the base of the second transistor is grounded. The units are interconnected such that the third terminal of each is coupled to the first terminal of the next adjacent unit in the loop. The second terminals of the units constitute the ports of the circulator.

United States Patent Beurrier [451 Oct. 24, 1972 N-PORT CIRCULATOR [73] Assignee: Bell Telephone Laboratories, Incorporated, Berkeley Heights, NJ.

[22] Filed: Aug. 19, 1971 [211 App]. No.: 172,967

[52] US. Cl. ..l79/l70 NC, 330/30 R, 333/11 [51] Int. Cl. ..H04b 3/36 [58] Field of Search ..179/170 R, 170 NC, 170 T;

Primary Examiner-Kathleen H. Clafly Assistant Examiner-William A. l-lelvestine Attorney-R. J. Guenther et a1.

[57] ABSTRACT This application describes an n-port circulator comprising n identical circuit units arranged in a closed loop configuration. Each unit is a three terminal circuit including two transistors and two equal, impedance-matching impedances. The first terminal is connected to the base of the first transistor, and to the emitter of the second transistor through one of the impedances. The second terminal is connected to the collector of the second transistor, and to the emitter of the first transistor through the other impedance. The third terminal is connected to the collector of the first transistor, while the base of the second transistor is grounded. The units are interconnected such that the third terminal of each is coupled to the first terminal of the next adjacent unit in the loop. The second terminals of the units constitute the ports of the circulator.

10 Claims, 7 Drawing Figures [56] References Cited UNITED STATES PATENTS 3,586,881 6/1971 Gaunt ..179/170 NC 2,885,492 5/1959 DHeedene ..l79/170R 3,513,401 5/1970 Tokunaga. ..322/167 3,612,780 10/1971 Beurrier ..179/170T PATENTED 3.700 832 sum 2 or 3 FIG. 5

. DIFFERENTIAL E AMPLIFIER 23 PATENTEDUBTZMQTZ 3,700 832 sum 3 or 3 FIG. 6

I N-PORT CIRCULATOR This invention relates to electromagnetic wave circulators.

BACKGROUND OF THE INVENTION Circulators are multiport nonreciprocal devices wherein wave energy is selectively coupled between pairs of ports in an ordered progression. Onetype of well-known circulator comprises either three or four wave paths, intersecting at a common junction wherein there is located a disc of magnetically biased gyromagnetic material. A second type of device comprises two, directionally coupled waveguides, one of which includes a gyromagnetic member. Because of their reliance upon gyromagnetic materials, however, the operating range of all of these devices is limited to the microwave frequencies. In addition, circulators having any arbitrary number of ports are not directly available, and can only be realized by various combinations of three and four port circulators. Finally, such prior art circulators are very large.

A second type of circulator, which avoids the limitations inherent in the type of circulators described hereinaboye, employs transistors, as described in an article titled Active circulators-The Realization of Circulators Using Transistors, by S. Tanaka et al., published in the Proceedings of the I.E.E.E, March 1965, pp. 260-267. (Also see Realization of the Circulator Concept Using Differential-Input Operational Amplifier, by A. W. Keen et al., published in Electronic Letters, September 1968, pp. 389-391.) The circulator to be described hereinbelow is of the second type, employing transistors and, as such, can be operated over an extended band of frequencies, including the audio frequencies. In addition, by utilizing only those circuit elements that can be fabricated by means of integrated circuit techniques, it can be made exceedingly small. Finally, it is a feature of this type of circulator that is can be readily made to have any arbitrary number of ports.

SUMMARY OF THE INVENTION An n-port circulator, in accordance with the present invention, comprises n identical circuit units arranged in a closed loop configuration. Each of these units is a three terminal circuit including two transistors and two equal, impedance-matching impedances. The first terminal is connected to the base of the first transistor, and to the emitter of the second transistor through one of the impedances. The second terminal is connected to the collectorof the second transistor and to the emitter of the first transistor through the other impedance. The third terminal is connected to the collector of the first transistor, while the base of the second transistor is grounded.

The units are interconnected such that the third terminal of each is coupled to the first terminal of the next adjacent unit in the loop. The second terminals of the units constitute the ports of the circulator.

Since no gyromagnetic material is used, the operating frequency is no longer confined to the microwave range. Indeed, a circulator, in accordance with the present invention can operate at any frequency for which transistors (or vacuum tubes) are available. Secondly, since only transistors, resistors and capacitors are used, the circulator can readily be constructed using integrated circuit techniques. Finally, circulators having any arbitrary number of ports can be realized simply by adding additional circuit units to the loop configuration.

It is also shown that each of the basic units has pseudo-circulator properties and can be used individually in those cases where complete circulator action is not required.

These and other objects and advantages, the nature of the present invention, and its various features, will appear more fully upon consideration of the various illustrative embodiments now to be described in detail in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a circulator in accordance with the present invention, employing n circuit units arranged in a loop configuration;

FIG. 2 shows the circuit details of the individual circuit units used in the circulator of FIG. 1;

FIG. 3 shows the use of an individual circuit unit as a pseudo circulator;

FIGS. 4 and 5 show a circuit unit modified to be matched at all three terminals;

FIG. 6 shows the use of two circuit units to form a telephone repeater circuit; and

FIG. 7 shows a complete circuit unit including the direct current bias portion of the circuit.

DETAILED DESCRIPTION Referring to the drawings, FIG. 1 shows, in block diagram, an n-port circulator comprising, in accordance with the present invention, n identical circuit units 10-1, 10-2, 10-(n-1), and 10-n arranged in a closed loop configuration. Each of the individual units, which will be described in greater detail hereinbelow, has three terminals, d, f, and g, and are connected such that terminal f of each unit is connected to terminal 3 of the next adjacent unit in the loop. The remaining terminal d of each unit constitutes a port of the circulator.

It is an advantage of the present invention that the number of ports can be arbitrarily increased simply by adding circuit units in the manner described. Thus, a three unit configuration makes a three-port circulator; five units make a five-port circulator; and, as illustrated, n units make an n-port circulator.

In operation, a signal applied at any one of the circulator ports is coupled through that unit to the circulator port of the next adjacent unit, as indicated by the arrows in each of the units, with none of the signal reaching any of the other circulator ports. The sequence of coupling, generally, is illustrated by the circulator arrow 8.

FIG. 2, now to be considered, shows the circuit details of the individual circuit units 10. To simplify the drawing, however, the bias portion of the circuit is not included. Thus, the signal portion of the circuit comprises two transistors 20 and 21, each of which has a base electrode b; an emitter electrode e; and a collector electrode 0. Terminal g is coupled to the base electrode of transistor 20 through a low impedance connection, and to the emitter electrode of a transistor 21 through an impedance-matching series impedance 22. Terminal d is coupled to the collector of transistor 21 through a low impedance connection, and to the emitter of transistor 20 through an impedance-matching .series impedance 23. Terminal f is coupled to the collector of transistor 20 through a low impedance connection, while the base of transistor 21 is grounded. (The circuit is similar to that disclosed in my copending application Ser. No. 113,200, filed Feb. 8, 1971, and assigned to applicants assignee.)

Itshould be noted that the term low impedance, as used herein means that the connection has a very low impedance. at the signal frequency (preferably a short circuit) relative to the magnitude of other relevant impedances in the circuit. For example, since the input impedance to the base of transistor 20 is very large, and since this is the relevant impedance in the input circuit of transistor 20, the inclusion of a relatively large impedance between terminal 3 and the base of transistor 20 (i.e., of the order of 0.1 the base impedance) would not materially affect the operation of the unit. On the other hand, the relevant impedance in the collector circuit of either transistor 21 or transistor 20 is the load impedance 2,. Since this is a relatively low impedance, of the order of a few hundred ohms, or less, the connection between the collector of transistor 21 and terminal d, and the collector of transistor 20 and terminal f should have a very low impedance, of the order of 0.1 2, or less. While the inclusion of any impedance in any of these wavepaths serves no useful purpose insofar as the present invention is concerned, in a practical situation it may be convenient to add some impedance in one or some of them as a means of equalizing the gains of the two transistors, as will be illustrated hereinbelow.

The operation of the circuit unit.l0 will now be explained by energizing each of the unit terminals g, d, and f in turn, while the other two terminals are suitably terminated. To illustrate this, a switching arrangement is shown connected to the respective terminals. For example with switches 5, 6, and 7 in switch position 1, terminal g is connected to a signal generator 24 having an output impedance Z, and an open-circuit voltage 2v. Simultaneously, terminals d and f are terminated by impedances 25 and 26.

As is known, the input impedances to the base and to the collector of a transistor are very high, whereas the input impedance to the emitter is verylow. In particular, in those practical cases of interest where the base and collector impedances are much larger than Z and the emitter impedance is much less than Z the former can be considered to be open-circuits, whereas the emitter can be considered to be a short circuit. So considered, generator 24, connected at terminal g, produces essentially no current flow into the base of transistor 20. The emitter, on the other hand, appears as a short circuit, producing an emitter current i given y i= 2v/2z, 1

o (2) The voltage at the base of transistor 20 is then given by v iZ The current into the emitter of transistor 21 produces a substantially equal current i in the collector of transistor 21, while the voltage v applied to the base of transistor 20 produces a substantially equal voltage v at the emitter of transistor 20. Designating the resulting emitter current in transistor 20 as i, and the current into load 25 as I, the following current and voltage relationships obtain:

vi'Z =IZ 4 and i i I 5 Noting, from Equation (2) that v 12,, we derive for I and i I i 6) 30 and Thus, a signal current i applied at terminal g is coupled to a load connected to terminal d. Since there is no emitter current in transistor 20, there is no collector current and no signal current is delivered to the load 26 connected to terminal f.

If, on the other hand, each of the switches 5, 6, and 7 are rotated to switch position 2, a signal source 27, having an output impedance Z, and an open-circuit voltage 2v, is coupled to terminal d. Simultaneously, signal source 24 is disconnected from terminal g and replaced by a terminating resistor 29 equal to Z,,. Terminal f remains terminated by impedance 26.

So terminated, signal source 27 sees an open circuit at the collector of transistor 21 and, hence, no current flows into transistor 21. However, the emitter of transistor 20 appears as a short-circuit, producing a current flow I (indicated by the dotted arrow) through impedance 23, and into transistor 20, given by o (9) This, in turn, produces an equal current flow I in load impedance 26 connected to terminal f. It will be further noted that no signal current is coupled to load impedance 29 connected to terminal g.

In switch position 3, each of the terminals 3 and d is terminated by an impedance Z,,, and a signal source 30 is connected to terminal f. The latter, however, is connected solely to the collector of transistor 20. Since the collector appears as an open circuit, no current is drawn, and no signal current is coupled to either of the other terminals g or d.

From the description of the operation of the circuit unit given hereinabove, the operation of the circulator illustrated in FIG. 1 is apparent. A signal applied to any of the circulator ports is coupled from terminal d of the corresponding circuit unit to terminal f of that unit. From there, the signal is coupled to terminal g of the next adjacent unit, and then through the unit to its terminal d, which constitutes the next adjacent circulator port. The signal flow is indicated by the arrows in the respective units.

It will also be noted from the description given above, that whereas the embodiment of FIG. 1 utilizes a plurality of circuit units to form a circulator, each of the individual circuits units has some of the characteristics of a circulator. For example, there is coupling between terminals g-d and d-f. However, there is no coupling between terminal f-g, which would be required of a true coupler. This latter failure, however, does not necessarily preclude using a signal circuit unit as a pseudo circulator, in many situations where complete circulator coupling is, in fact, not desirable. One such use is illustrated in FIG. 3.

In the application depicted in FIG. 3, terminal g of circuit unit 30, of this type shown in FIG. 2, is connected to a signal transmitter 31 in a telephone handset 35. A two-way transmission link 32 is connected to terminal d and a signal receiver 33 in handset 35 is connected to terminal f. In operation, the output signal from the transmitter is coupled from terminal g to terminal d and transmission link 32. Any oppositely propagating signal, on the other hand, is coupled between terminals d and f to receiver 33. There is no coupling between terminals f and g, and none is either needed or desired.

From the circuit diagram of FIG. 2, it is noted that a load or a source connected at either terminals g or d is matched by the series impedances 22 or 23, respectively. However, there is no corresponding matching impedance at terminal f. Indeed, the circuit unit has a very high input impedance, equal to the collector impedance of transistor 20, at terminal f. One way of correcting this is to add a shunting impedance Z, to the collector. This, however, would reduce the output at terminal f by one-half and is, therefore, an unsatisfactory solution to the problem. A first alternative arrangement for producing a match at terminal f that does not result in any loss of signal is illustrated in FIG. 4.

The embodiment of FIG. 4 shows the basic circuit unit, comprising transistors 20 and 21, and series impedances 22 and 23, to which there has been added an arbitrary load impedance 45 of magnitude Z,,, a 1:N turns ratio transformer 40, a matching impedance 41, and a third transistor 42. More particularly, the primary winding 43 of transformer 40 is connected across impedance 23. The secondary winding 44 of transformer 40 is connected between the base electrode of transistor 42 and ground. Impedance 41 is connected between the emitter electrode of transistor 42 and the collector of transistor 20. The collector electrode of transistor 42 is grounded.

In operation, a signal source 27 produces a current I in impedance 23, as explained hereinabove in connection with FIG. 2. This, in turn, results in a voltage v I 2,, (l0) across the primary winding 43 of the transformer. Being a l:N turns ratio transformer, a voltage v'N, induced across the secondary winding 44, is coupled to the base of transistor 42 and from there to the emitter. Thus, the flow of current through impedance 23 results in a voltage vN being produced at one end of impedance 41.

Designating the currents through impedance 41 and load impedance 26 connected to terminal f as i and i, respectively, and noting that the collector current in transistor 20 is I', the following current and voltage relationship obtain:

I i= i' 11 and v'N-iZ,,'=i'Z,,' (I2) Substituting for v from Equation and solving for i and i', we further obtain that i=I' 13) and z=0 (14) when N=Z,,'/Z,, (15) Thus, in the arrangement of FIG. 4, all of the current i coupled into terminal d by source 27, is delivered to the load 45 and none is lost in matching impedance 41. Nevertheless, looking into terminal f one sees a matching impedance 41 in shunt with the collector of transistor 20. Thus, in the embodiment of FIG. 4, the circuit unit is matched at all three terminals.

FIG. 5 shows a second embodiment of a circuit unit matched at all three terminals. This circuit is identical in all respects to that illustrated in FIG. 4 except that transformer 40 is replaced by differential amplifier 50 whose output voltage is equal to v'G, where the amplifier gain G Z,,/Z,,. The advantage of this latter arrangement is that it does not use transformers and, hence, is readily intergrable.

FIG. 6, now to be considered, shows the use of two circuit units for forming the familiar telephone repeater circuit. As illustrated, circuit units and 61 are connected, as in FIG. 1, with terminal f of each unit connected to terminal g of the other unit. Terminal d are the two repeat ports A and B. In this configuration, however, an amplifier, adapted to amplify signals propagating in the direction from f to g, is included in each of the interunit connections. Thus, a first amplifier 62 is included between terminal f of unit 60 and terminal g of unit 61, and a second amplifier 63 is included between terminal f of unit 61, and terminal g of unit 60.

In operation, a signal applied at repeated port A is coupled to repeater port B via amplifier 62, whereas a signal applied at repeated port B, is coupled to port A via amplifier 63.

FIG. 7 is included to show an actual circuit unit, including the direct current bias portion of the circuit, designed to operate at audio frequencies with 600-ohm loads.

Using the same identification numerals as in FIG. 2, the unit includes transistors 20 and 21, and series impedances 22 and 23. It will be noted that each terminal is coupled to the unit through a 40 if capacitor. It will' also be noted that series impedance 22 includes a 500 Q potentiometer, and that the base of transistor 20 is connected to the adjustable tap of the potentiometer. This is done to equalize the gains of the two transistors. As a result, some small resistance is included in the connection between terminal g and the base of transistor 20. However, since this resistance, and the impedance of the 40 uf coupling capacitance at the frequency of interest are much smaller than the base impedance, for all practical purposes, the connection between terminal g and the base of transistor 20 is a low impedance connection. Similarly, the connections between terminal d and the collector of transistor 21, and between terminal f and the collector of transistor 20 through the 40 uf coupling capacitors are low impedance connections at the frequency of interest.

The added circuit components, including resistors and capacitors serve to establish the necessary direct current bias conditions, and to provide low impedance signal paths around the bias circuits.

It will be recognized that amplifiers can be included between pairs of adjacent terminals f and g in the embodiment of FIG. 1 just as they were included in the repeater circuit of FIG. 6. Thus, a circulator, in accordance with the present invention, can be utilized to provide gain as well as isolation and signal directing. It will also be recognized that whereas the illustrative embodiments employ transistors, any active elements having an emitting electrode, a control electrode and an emission collector electrode, such as vacuum tubes, can just as readily be used in each of the circuit units. Thus, in all cases it is understood that the above described arrangements are illustrative of but a small number of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can readily be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.

lclaim:

1. A three terminal, pseudo circulator comprising:

first and second active elements, each of which has an emitting electrode, a control electrode and an emission collector electrode;

the first terminal of said circulator being connected to the control electrode of said first active elementand to the emitting electrode of said second active element through a first series impedance;

the second terminal of said circulator being connected to the collector electrode of said second element and to the emitting electrode of said first element through a second series impedance;

the control electrode of said second element being grounded;

and the third terminal of said circulator being ,con-

nected to the collector electrode of said first element.

2. The combination according to claim 1 including:

a signal transmitter connected to said first terminal;

a two way transmission link connected to said second terminal;

and a signal receiver connected to said third terminal.

3. The combination according to claim 1 wherein:

said first and second impedances are equal and provide an impedance match for external loads connected to said first and second terminals.

4. A two way circuit comprising:

a pair of pseudo circulators in accordance with claim 1, including:

means for coupling the third terminal of each circulator to the first terminal of the other circulator.

5. The circuit according to claim 4, wherein:

each of said coupling means includes an amplifier.

6. An n-port circulator comprising: n identical three terminal circuit units, each of which includes: first and second transistors, each having an emitter electrode, a base electrode and a collector electo the collector electrode of said second transistor and to the emitter electrode of said first transistor through a second series impedance; the third terminal of each unit being connected to the collector electrode of said first transistor; the base electrode of said second transistor being grounded; said units being connected in a loop configuration, with the third terminal of each unit being coupled to the first terminal of the next adjacent unit in said loop;

and the second terminal of said n units being the n ports of said circulator.

7. The circulator according to claim 6 wherein the third terminal of each unit is coupled to the first terminal of the next adjacent unit by means of an amplifi- 8. The circulator according to claim 6 wherein each unit includes, in addition;

a third transistor having an emitter electrode, a base electrode and a collector electrode;

the collector of said third transistor being grounded;

the emitter of said third transistor being connected to the collector of said first transistor through a third series impedance;

and means for coupling the voltage across said second impedance to the base electrode of said third transistor.

9. The circulator according to claim 6 wherein each unit includes, in addition:

a differential amplifier having an output terminal and two input terminals;

said output terminal being connected to the collector of said first transistor through a third series impedance;

and means for connecting opposite ends of said second series impedances to a different one of said input terminals. 10. The circulator according to claim 9 wherein; said three series impedances are equal, and provide an impedance match for equal external loads connected to the terminals of said units. 

1. A three terminal, pseudo circulator comprising: first and second active elements, each of which has an emitting electrode, a control electrode and an emission collector electrode; the first terminal of said circulator being connected to the control electrode of said first active element and to the emitting electrode of said second active element through a first series impedance; the second terminal of said circulator being connected to the collector electrode of said second element and to the emitting electrode of said first element through a second series impedance; the control electrode of said second element being grounded; and the third terminal of said circulator being connected to the collector electrode of said first element.
 2. The combination according to claim 1 including: a signal transmitter connected to said first terminal; a two way transmission link connected to said second terminal; and a signal receiver connected to said third terminal.
 3. The combination according to claim 1 wherein: said first and second impedances are equal and provide an impedance match for external loads connected to said first and second terminals.
 4. A two way ciRcuit comprising: a pair of pseudo circulators in accordance with claim 1, including: means for coupling the third terminal of each circulator to the first terminal of the other circulator.
 5. The circuit according to claim 4, wherein: each of said coupling means includes an amplifier.
 6. An n-port circulator comprising: n identical three terminal circuit units, each of which includes: first and second transistors, each having an emitter electrode, a base electrode and a collector electrode; the first terminal of each unit being connected to the base electrode of said first transistor and to the emitter electrode of said second transistor through a first series impedance; the second terminal of said unit being connected to the collector electrode of said second transistor and to the emitter electrode of said first transistor through a second series impedance; the third terminal of each unit being connected to the collector electrode of said first transistor; the base electrode of said second transistor being grounded; said units being connected in a loop configuration, with the third terminal of each unit being coupled to the first terminal of the next adjacent unit in said loop; and the second terminal of said n units being the n ports of said circulator.
 7. The circulator according to claim 6 wherein the third terminal of each unit is coupled to the first terminal of the next adjacent unit by means of an amplifier.
 8. The circulator according to claim 6 wherein each unit includes, in addition; a third transistor having an emitter electrode, a base electrode and a collector electrode; the collector of said third transistor being grounded; the emitter of said third transistor being connected to the collector of said first transistor through a third series impedance; and means for coupling the voltage across said second impedance to the base electrode of said third transistor.
 9. The circulator according to claim 6 wherein each unit includes, in addition: a differential amplifier having an output terminal and two input terminals; said output terminal being connected to the collector of said first transistor through a third series impedance; and means for connecting opposite ends of said second series impedances to a different one of said input terminals.
 10. The circulator according to claim 9 wherein; said three series impedances are equal, and provide an impedance match for equal external loads connected to the terminals of said units. 